CELT

Constrained Energy Lapped Transform
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.wikipedia
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Opus (audio format)

OpusOpus audio format.opus
Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).
Opus combines the speech-oriented LPC-based SILK algorithm and the lower-latency MDCT-based CELT algorithm, switching between or combining them as needed for maximal efficiency.

Lossy compression

lossylossy data compressioncompressed
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Ogg

OGM.oggOgg Writ
Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).
Being a container format, Ogg can embed audio and video in various formats (such as Dirac, MNG, CELT, MPEG-4, MP3 and others) but Ogg was intended to be, and usually is, used with the following Xiph.org free codecs:

Modified discrete cosine transform

MDCTModulated Lapped Transformtime-domain aliasing cancellation
CELT is a transform codec based on the modified discrete cosine transform (MDCT) and concepts from CELP (with a code book for excitation, but in the frequency domain).
It is employed in most modern audio coding standards, including MP3, Dolby Digital (AC-3), Vorbis (Ogg), Windows Media Audio (WMA), ATRAC, Cook, Advanced Audio Coding (AAC), LDAC, Dolby AC-4, MPEG-H 3D Audio, as well as speech coding standards such as AAC-LD (LD-MDCT), G.722.1, G.729.1, CELT, and Opus.

SILK

Skype SILK
Development is going on only for its hybridised form as a layer of Opus, integrated with SILK.
SILK is a foundation (with CELT) of the hybrid codec Opus (at the time called "Harmony") that was submitted to the IETF in September 2010, and was chosen as the final candidate for the new standard.

Mumble (software)

MumbleMumble room
Despite the format not being finally frozen, it was being used in many VoIP applications such as Ekiga and FreeSWITCH, which switched to CELT upon entering soft-freeze in January 2009, as well as Mumble, TeamSpeak and other software.
Mumble uses the low-latency audio codec Opus as of version 1.2.4, the codec that succeeds the previous defaults Speex and CELT.

Xiph.Org Foundation

Xiph.orgXiphXiph QuickTime Components
Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).

Code-excited linear prediction

CELPCode Excited Linear PredictionCode-excited linear prediction (CELP)
CELT is a transform codec based on the modified discrete cosine transform (MDCT) and concepts from CELP (with a code book for excitation, but in the frequency domain). It borrows ideas from the CELP algorithm, but avoids some of its limitations by operating in the frequency domain exclusively.

Ekiga

GnomeMeeting
Despite the format not being finally frozen, it was being used in many VoIP applications such as Ekiga and FreeSWITCH, which switched to CELT upon entering soft-freeze in January 2009, as well as Mumble, TeamSpeak and other software.

FreeSWITCH

FreeSWITCH IPBX
Despite the format not being finally frozen, it was being used in many VoIP applications such as Ekiga and FreeSWITCH, which switched to CELT upon entering soft-freeze in January 2009, as well as Mumble, TeamSpeak and other software.

Voice over IP

VoIPVoice over Internet Protocolvoice-over-IP
Despite the format not being finally frozen, it was being used in many VoIP applications such as Ekiga and FreeSWITCH, which switched to CELT upon entering soft-freeze in January 2009, as well as Mumble, TeamSpeak and other software.
The same year, a DCT algorithm was the basis MDCT has since been widely used in VoIP applications, such as the G.729.1 wideband codec introduced in 2006, Apple's Facetime (using AAC-LD) introduced in 2010, the CELT codec introduced in 2011, the Opus codec introduced in 2012, and WhatsApp's voice calling feature introduced in 2015.

Royalty-free

royalty free
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Data compression

compressionvideo compressioncompressed
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Free software

freefree-softwarefreely
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Audio codec

audio codecscodecaudio
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Latency (engineering)

latencylow latencylow-latency
Constrained Energy Lapped Transform (CELT) is an open, royalty-free lossy audio compression format and a free software codec with especially low algorithmic delay for use in low-latency audio communication.

Software patent

software patentspatentpatents
The algorithms are openly documented and may be used free of software patent restrictions.

Internet Engineering Task Force

IETFInternet Engineering Task Force (IETF)IETF Working Group
Development of the format was maintained by the Xiph.Org Foundation (as part of the Ogg codec family) and later coordinated by the Opus working group of the Internet Engineering Task Force (IETF).

Vorbis

Ogg VorbisOGGaoTuV
CELT was meant to bridge the gap between Vorbis and Speex for applications where both high quality audio and low delay are desired. Like Vorbis, CELT is a fullband (entire human hearing range) general-purpose codec, i.e. not specialized for special types of audio signals and therefore different from its sibling project Speex.

Speex

SPX.spxlibspeex
CELT was meant to bridge the gap between Vorbis and Speex for applications where both high quality audio and low delay are desired. Like Vorbis, CELT is a fullband (entire human hearing range) general-purpose codec, i.e. not specialized for special types of audio signals and therefore different from its sibling project Speex.

Frequency domain

frequency-domainFourier spaceFourier domain
It borrows ideas from the CELP algorithm, but avoids some of its limitations by operating in the frequency domain exclusively.

Sampling (signal processing)

sampling ratesamplingsample rate
It can use a sampling rate from 32 kHz to 48 kHz and above and an adaptive bitrate from 24 kbit/s to 128 kbit/s per channel and above.

BSD licenses

BSD licenseBSDNew BSD License
There are no known intellectual property issues pertaining to the CELT algorithm, and its reference implementation is published under a permissive open-source license (the 2-clause BSD).

Hearing range

audible rangepitch perceptionrange of human hearing
Like Vorbis, CELT is a fullband (entire human hearing range) general-purpose codec, i.e. not specialized for special types of audio signals and therefore different from its sibling project Speex.